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Full text of "Handbook Of Chemical Engineering - I"

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LEACHING AND DISSOLVING
343
In Fig. 2 are given two curves taken from some tests on leaching copper ore with ferric sulphate which corresponds to the theoretical curve of Fig. 1. Curves A and B were obtained by "plotting time against percentage of extraction. As the tests
100
g so
•   60
,40
| 20
1.00
3.00
2. CO Time FIG. 1.—Rate of solution.
4.00
were made by intermittent agitation the percentage extraction here corresponds to the concentration of solution in Fig. 1, the concentration of the solution is directly dependent upon the amount of solids extracted. Curve A was made on one type of ore and B on another and yet the only difference in the curve is a vertical displacement due to the difference in the relative rates of dissolution.
40
0                      24                     48                     72                     96
Time-hrs. FIG.  2.—Copper extraction tests.
If we allow the process of dissolution to continue without setting up any artificial currents to aid diffusion this concentrated film in direct contact with the solid is going to become thicker and thicker and the speed of dissolution is going to become slower and slower as the thickness increases. This is not due to the effect of mass action alone but also because it takes a definite period of time for the dissolved particles to